Production of powder alloy products

ABSTRACT

METHOD OF PRODUCING DISPERSION STRENGTHENED SUPERALLOYS HAVING IMPROVED HIGH STRESS-REPTURE PROPERTIES AT ELEVATED TEMPERATURES WHEREIN A POWDER METALLURGY PRODUCT PROCESSED SO AS TO ELIMINATE FINE GRAINS, IS HOT WORKED TO PRODUCE A METALLURGICAL STRUCTURE CHARACTERIZED BY COARSE, ELONGATED GRAINS.

United States Patent ()flice 3,814,635 Patented June 4, 1974 3,814,635 PRODUCTION OF POWDER ALLOY PRODUCTS Dante James Cometto, Huntington, W. Va., Jeremy Painter Morse, Chesapeake, Ohio, and Robert Edward Crickmer and Irvin Kirk, Huntington, W. Va., assignors to The International Nickel Company, Inc., New York, N.Y. No Drawing. Filed Jan. 17, 1973, Ser. No. 324,313 Int. Cl. C22f J/OO, 1/10, l/ll US. Cl. 148-115 R 13 Claims ABSTRACT OF THE DISCLOSURE Method of producing dispersion strengthened superalloys having improved high stress-rupture properties at elevated temperatures wherein a powder metallurgy product processed so as to eliminate fine grains, is hot worked to produce a metallurgical structure characterized by coarse, elongated grains.

The present invention relates to powder metallurgy, and more particularly to the production of dispersion strengthened, mechanically alloyed superalloys characterized by an improved combination of characteristics at elevated temperature, e.g., about 1800 F. and above.

Notwithstanding the significant achievements which have been made over the years in respect of materials capable of delivering improved metallurgical properties, extensive research efforts continue in the quest for new alloys capable of meeting the ever increasing demands imposed by advanced designs, higher temperatures, increased loading stresses, etc. Recently, in this regard, a significant breakthrough was achieved by reason of the mechanical alloying concept as described in US. Pat. No. 3,591,362 (incorporated herein by reference). This unique process involves the dry, intensive, high energy milling of powders such that the constituents are comminuted and cold bonded by the continuous and repetitive impacting action of attritive elements whereby composite product powder particles of substantial saturation hardness are obtained, the composition of which corresponds to the percentages of the respective constituents in the original charge. By virtue of such high energy milling, the initial constituent powders become most intimately interdispersed at close interparticle spacing, the composite particles being exceptionally dense and homogeneous. As a consequence, superalloys so produced exhibit an exceptional level of high temperature characteristics.

Given the attributes of mechanically alloyed superalloys, it has nonetheless now been found that a still further marked improvement in high temperature stress rupture characteristics can be obtained. In accordance herewith, this involves the application of a special sequence of processing operations, including grain coarsening and working, capable of providing superalloys having stable, elongated grain structures substantially free of fine grains. In this connection, the presence of fine grains that might otherwise be present in mechanically alloyed superalloys is virtually eliminated and this is decidedly beneficial since fine grains detract from high temperature strength. The invention permits of the production, with virtually unlimited working reduction in thickness, of mill products, including sheet, plate, wire, tubing, etc., of enhanced utility.

Generally speaking, the subject invention involves (a) hot consolidating, as by extrusion, superalloy powder, particularly mechanically alloyed powder containing a fine, well distributed, inert dispersoid, under (i) conditions of temperature, reduction ratio, and strain rate correlated (ii) to provide a solid product having a fine grain, e.g., having dimensions less than about microns and such that upon (b) heating the solid product to a grain coarsening temperature of about 1200 C to 1350 C. or higher but below the incipient melting temperature, (iii) a coarse grain structure, e.g., a grain structure having a minimum value of the smallest grain dimension of at least about 10 microns and having an average value of the smallest grain dimension of at least about 200 microns, i.e., a grain structure essentially free of fine grains, is produced. This processing is then followed by (c) hot working the consolidated grain-coarsened product to achieve a hot worked product characterized by coarse, elongated grains having an aspect ratio of greater than 8: 1, e.g., 16:1 and higher, whereby substantially improved stress-rupture properties are attained as compared with the properties of the material in the initially graincoarsened condition.

As will be further described herein, solid products obtained by hot extrusion can, prior to initial grain coarsening, be beneficially further hot worked, as by, for example, hot rolling or swaging and the like. In any case, the condition of the resultant solid product should be such that, when subjected to a subsequent high temperature grain coarsening heat treatment, essentially all of the resulting grains measure greater than about 10 microns in their smallest dimension. It is preferred that the grains after this treatment have an average value for their smallest dimension of at least 200 microns, e.g., 1000 microns for more. It has been found, in solid products which have been given the grain coarsening heat treatment, that the lower the average aspect ratio of the coarsened grain, the greater is the likelihood that a grain structure has been obtained which is essentially free of fine grains, the aspect ratio being the ratio of the maximum dimension of a grain to the minimum dimension of the grain. Generally, insofar as we are aware, the consideration heretofore has generally been to achieve high aspect ratios by this type of grain coarsening treatment, say greater than 10:1 and up to 50:1 or more. It is deemed beneficial that the average grain aspect ratio imparted by the initial grain coarsening not exceed about 4:1. A ratio of up to about 8:1 is contemplated; however, it is considered marginal.

In terms of alloys in general, the present invention is applicable to a wide variety of dispersion-strengthened metal systems, and is particularly advantageous for producing dispersion-strengthened superalloys containing, by weight, up to about chromium, up to about 8% aluminum, up to about 8% titanium, up to about 40% molybdenum, up to about 40% tungsten, up to about 20% columbium, up to about 30% tantalum, up to about 40% copper, up to about 3% vanadium, up to about 15% manganese, up to about 2% carbon, up to about 2% silicon, up to about 1% boron, up to about 2% zirconium, up to about 0.8% magnesium, up to about 4% hafnium, up to about 10 volume percent refractory dispersoid material, the balance being essentially at least one iron group metal (iron, nickel, cobalt) with the sum of the iron group metals being at least 25%.

The melting point of the superalloys are desirably in the temperature range of about 1200 C. to about 1375 C. or higher. Preferably, the superalloys contain about 5 to about 50 to 60%, e.g., about 5 to 35%, chromium; up to about 6.5%, e.g., about 0.2 to about 5%, aluminum; up to about 6.5%, e.g., about 0.2 to about 5%, titanium; up to about 10 or 15%, e.g., up to about 4%, molybdenum; up to about 20%, e.g., up to about 6%, tungsten; up to about 10%, columbium; up to about 10%, e.g., up to about 3%, tantalum; up to about 2%, vanadium; up to about 2% manganese; up to about 1% silicon; up to about 0.75% carbon; up to about 0.1% boron; up to about 1% zirconium; up to about 0.2% magnesium, up to about 40%iron, up to about 5% or 7% by volume of at least one refractory dispersoid material having an average particle size of 3 about to 100 millimicrons; and the balance essentially nickel in an amount at least about 40% of the composition. The nickel can be replaced in Whole or in part by cobalt.

Refractory compounds that can be used as the dispersoid include refractory oxides, carbides. nitrides, borides, notably the oxides, carbides, nitrides and borides of such refractory metals as yttrium, lanthanum and thorium. Other refractory oxides such as those of zirconium, titanium, cerium, beryllium, aluminum and the like may be utilized. The refractory oxides generally include the oxides of those metals whose negative free energy of formation of the oxides per gram-atom of oxygen at about 25 C., is about 90 kcal. or more negative and whose melting point is at least about 1300 C. Dispersoid materials that are particularly useful include yttria, lanthana, ceria, zirconia and thoria in sizes smaller than about one micron and advantageously, smaller than 0.1 micron.

It is deemed beneficial that the powder be mechanically alloyed beyond the milling time required to attain saturation hardness for the composition to provide compositional homogeneity or uniformity, including substantially uniform distribution of finely-divided dispersoid materal therethrough. The homogeneity of the powder should be such that when a polished section is viewed at 250 diameters, initial constituents of the powder mix cannot be identified. It is generally advantageous that the mechanically alloyed composite powders range in size from about 3 microns to about 200 microns, more preferably, about 20 microns to about 200 microns, it being possible for the powder particles to range in size up to about 500 microns.

By way of illustration in respect of the foregoing, where the final material is of a nickel-base superalloy composition containing, by weight, up to about 25%, e.g., about to 22%, chromium; up to about 5%, e.g., about 2 to 3%, titanium; up to about 5%, e.g., 1 to 2%, alumminum; up to about 2%, e.g., 0.5 to 2% (volume) of a dispersoid such as yttria; the balance essentially nickel, it is considered advantageous to carry out the mechanical alloying operation in a Szegvari attritor of, for example, IOU-gallon capacity, with a charge of steel grinding balls sufiicient to provide a ball-to-powder weight ratio of about 5 to l to about 30 to 1, preferably about 7.5 to 1 to about 20 to 1, for about 5 to about 40 hours, preferably about 7 to 25 hours, at an impeller speed of about 70 to about 150 r.p.m., with about 75 to about 140 r.p.m. being preferred. It is generally preferred that the mechanical alloying operation be conducted for longer times where a lower impeller speed is employed in order to provide homogeneous powder. Other factors including the particle size and composition of constitutent powders in the initial powder mixture can affect the milling time required to provide powders of homogeneous structure and composition. While it has been found beneficial to mechanically alloy nickel-base superalloy compositions under a flowing atmosphere mixture of, for example, nitrogen with about 0.25% air, an atmosphere of sealed air, or an atmosphere of other gas mixtures may also be used, e.g., argon and air. Interdispersion cold bonding agents as described in U.S. applications Ser. Nos. 327,321 and 327,323, both filed Jan. 29, 1973, can also be employed.

As to hot consolidation of mechanically alloyed superalloy powders, the most easily controllable parameters are temperature and reduction ratio. Strain rate, another important parameter, is a function of power available in the consolidation equipment employed. As indicated above, this consolidation step can be carried out by hot extrusion. It is generally desirable that the extrusion be conducted within the range of about 900 C. to about 1150 C., at a ratio of not less than about 4:1, preferably at least 5:1 or higher, and a relatively high strain rate. In the case of hot extrusion, strain rate is directly proportional to ram speed.

The requisite grain structure essentially free of fine grains is obtained in an extruded product which has been given the high temperature grain coarsening heat treatment without any additional hot-working prior'to heat treatment when, with reference to an extrusion having a 9-inch diameter container, the extrusion temperature is maintained between about 900 C. and about 1066" C., the extrusion ratio is maintained between about 5 :1 to about 50:1 and the ram speed exceeds a minimum required value which depends upon the temperature and ratio. The minimum ram speed required for different extrusion temperatures and ratios is given in Table I in respect of how these parameters may be correlated with regard to the extrusion of mechanically alloyed superalloy powders such as those which respond in similar fashion to an alloy nominally containing about 20% chromium, about 2.5% titanium, about 1.5% aluminum, about 1.3% yttria, the balance essentially nickel (Alloy A).

The combinations of extrusion parameters set forth in Table I are generally operative to yield the desired grain structure essentially free of grain in solid products of such composition after the high temperature grain coarsening heat treatment, but some departure from these combinations is permissible. Thus, when one parameter is outside the range given, another parameter can be adjusted so as still to arrive at the required product. The important points to be recognized are that higher extrusion temperatures or lower extrusion ratios require higher ram speeds and that low aspect ratios are a reflection of the absence of detrimental fine grains. The latter also reflects that the stress rupture properties of the alloys at this point in processing are, in a sense, deliberately sacrificed. However, this is obviated by the subsequent hot working step.

TABLE I Minimum ram speed (in/sec.)

Extru- 9-in. 9-in. 14-in. sion diameter diameter diameter Extrusion temp. 0.) ratio container container container Should the requirements as set forth in Table I be beyond the capability of the particular extrusion equipment available, the solid product resulting from extrusion can be subjected to further hot-working in a separate operation before the grain coarsening treatment in order to obtain the required structure. Such subsequent operations may include swaging, rolling, forging, etc. Just as with extrusion, parameters of this hot-working step must be correlated. Hot working temperature, reduction, and rate of reduction must be balanced, so as to obtain a product having a fine grain but such that when the solid product is heated to the grain coarsening temperature, a coarse grain structure is formed which is essentially free of fine grams.

As an alternative to extrusion, the mechanically alloyed powder can be consolidated by other means, for example, by hot pressing or hot rolling. In such operations, strain rates and reduction ratios are lower than in hot extrusion. When such alternative means are resorted to, the resulting consolidated product is subjected to further working in a separate operation, e.g., swaging, rolling, etc., before the grain coarsening heat treatment to develop grain structure which is essentially free of fine grains.

The solid superalloy product will usually have a fine grain size, e.g., less than 10 microns. It is then heated to an elevated temperature below the incipient melting point to achieve grain coarsening. For substantially complete grain coarsening, the solid product is heated at a temperature of about 1200 C. or higher, e.g., about 1220 to about 1345 C., but below the incipent melting point, for the requisite time, e.g., about one-half hour.

When the necessary coarse grain structure essentially free of fine grain has been produced, the solid product is then hot worked to mechanically transform the grains to coarse, elongated shape so that the grains in the final product have a high aspect ratio of about 8:1 to 64:1 or higher. The working of the grain coarsened product preferably is carried out at an elevated temperature, e.g., about 980 C. to about 1200" C. to reduce the product by an amount suflicient to produce the desired aspect ratio. Hot working temperatures, reductions, and reduction rates should be correlated, so as to prevent any substantial metallurgical transformation to fine grain.

An outstanding advantage of the present invention is that the grain coarsened product can besubjected to large hot reductions, e.g., reductions of up to 75% or 90% 'or even higher, to provide the desired coarse, elongated grains and, consequently, superior elevated temperature properties. This capability is an important and significant advantage in operations for producing sheet, foil, plate, bars, etc., where large amounts of hot reduction provide cost savings and other advantages. Such working advantageously can be carried out by hot rolling the grain coarsened product either unidirectionally, as by rolling in a single direction, or multidirectionally, as by cross-rolling, for example. Rolling in a single direction generally results in a fibrous grain morphology, while two-directional rolling produces plate-like grains, when viewed two-dimensionally. This hot working step can also be accomplished by hot forging or other methods.

Using Alloy A for purposes of illustration, hot working the grain coarsened product to produce coarse, elongated grains therein, is carried out at a temperature in the range of about 980 C. to about 1200 C., preferably, about 1065 C. to about 1180 C., e.g., about 1090 C. to 1150 C. to achieve a working reduction of about 35% or more, preferably about 50% or more, e.g., about 75%. Where the hot working of the grain coarsened product is carried out by rolling in a number of passes, it is of advantage to employ a reduction per pass of less than about 40% and preferably less than about 30%, e.g., not greater than about 25% of the cross-sectional area of the grain coarsened product.

The hot rolled products should exhibit virtually no fine grains at magnifications as high as 1000 diameters, the absence thereof contributing to the consistently superior high temperature properties of products produced in accordance herewith.

The uniform distribution of the dispersoid in the solid products produced according to the present invention remains substantially unaffected by the grain-coarsening heat treatment and the subsequent hot working procedure above described, there being no evidence of agglomeration of the dispersoid particle in the product at magnifications up to 20,000 diameters even after the grain coarsening heat treatment at 2400,'F. for one-half hour and notwithstanding the large size of the resulting coarsened grains.

To afford those skilled in the art a better appreciation of the invention, the following is given:

EXAMPLE I A 36.25 kg. powder charge containing 24.86 kg. of nickel powder of. 4 micron size, 7.52 kg. of 6 micron chromium powder, 3.37 kg. of -200 mesh nickel-titanium-aluminum master alloy, and 0.50 kg. of 28 millimicron yttria was mechanically alloyed for hours under a flowing atmosphere, consisting of nitrogen with about 0.25% air in a 100-gallon capacity Szegvari attritor operated at an impeller speed of 98 rpm. and with 1600 pounds of inch diameter steel balls providing a ballto-powder ratio of 20:1, to produce compositepowder particles of Alloy A that were substantially homogeneous in composition. The composite powder was filled into a 8%-ll'l0h diameter mild steel can, which was then sealed to air. The canned composite powder was then heated to a temperature of about 980 C. and extruded, with an extrusion ram speed of 2.5 inches per second in a press having a 9-inch diameter container at a 16:1 extrusion ratio.

A portion of the extrusion thus produced was grain coarsened by heat treating it at 1315 C. for one-half hour and metallographic examination thereof indicated the grain coarsened product to have an average grain aspect ratio of 8:1 (on the high side) and to be free of fine grain. A specimen cut from the grain coarsened product was stress-rupture tested at 1038 C. under a load of 17,500 p.s.i., the resulting rupture life being 26.9 hours. This value is quite good and is representative of the prior art for this alloy.

Another portion of the extrusion was also heat treated (initially grain coarsened) at 1315 C. for one-half hour but was then hot rolled on a mill using 14-inch diameter rolls at a roll speed of rpm. and at 1120" C. to achieve a 75% reduction therein, 10 passes being employed with the following reduction schedule: 28.6%, 5.5%, 6.4%, 9.9%, 6.9%, 9.6%, 6.5%, 20.2%, 15.0% and 18.0%. Specimens from the hot rolled piece (which had an average grain aspect ratio of about 64:1) were then stress-rupture tested at the same 1038 C. temperature, but under a much higher load of 22,000 p.s.i. The resulting stress-rupture life was 21.6 hours. The 4,500 p.s.i. improvement in elevated temperature strength brought about by the present invention is quite dramatic, the resulting stress-rupture strength far exceeding the acceptance criterion of a 1038 C. stress-rupture life of hours at the much lower stress of 16,000 p.s.i.

The product provided in accordance with the invention is useful in the production of articles such as gas turbine blades and vanes and other articles subjected in use to the combined effects of elevated temperature and stress.

While the invention as above described has been primarily addressed to mechanically alloyed powders, it would also be applicable to other powders concerning which a comparable amount of strain energy had been imparted.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for improving elevated temperature stress rupture properties of products made from mechanically alloyed powder which comprises hot consolidating the powder to provide a solid product having fine grains, heating the solid product at a temperature above about 1200" C. but below the incipient melting point of the alloy to achieve a coarse grain structure free of detrimental fine grains and characterized by a relatively low average grain aspect ratio, and thereafter hot working the coarse grained solid product to achieve a product characterized by coarse, elongated grains, the structure remaining free of deleterious fine grains.

2. A process in accordance with claim 1 in which the fine grains of the hot consolidation step have dimensions not greater than about 10 microns on average.

3. A process in accordance with claim '1 in which the smallest grain dimension of the initially coarsened structure is at least about 10 microns and the average value of the smallest grain dimension is at least about 200 microns.

4. A process in accordance with claim 1 in which average grain aspect ratio of the initially coarsened structure is less than 8:1.

5. A process in accordance with claim 4 in which the aspect ratio does not exceed about 4: 1.

6. A process in accordance with claim 1 in which the aspect ratio of the coarse elongated grains produced by the hot working step is greater than 8: 1.

7. A process in accordance with claim 6 in which the said aspect ratio is at least about 12:1.

8. A process in accordance with claim 1 in which subsequent to hot consolidation and prior to initial grain coarsening the solid product is subjected to hot Working.

9. A process in accordance with claim 1 in which hot consolidation is accomplished by extrusion of the alloy powder.

10. A process in accordance with claim 9 in which the said hot Working step is conducted within the temperature range of about 980 to about 1200 C.

11. A process in accordance with claim 9 in which the working reduction is at least about 35% 12. A process in accordance with claim 1 in which the alloy product contains up to about 65% chromium, up to about 8% aluminum, up to about 8% titanium, up to about 40% molybdenum, up to about 40% tungsten, up to about 20% columbium, up to about 30% tantalum, up to about 40% copper, up to about 3% vanadium, up to about 15% manganese, up to about 2% silicon, up to about 2% carbon, up to about 1% boron, up to about 2% zirconium, up to about 0.57 magnesium, up to about 4% hafnium, up to about 10 volume percent refractory dispersoid material and the balance at least one element from the group consisting of nickel, cobalt and iron in an aggregate amount of at least 25%.

13. A process in accordance with claim 12 in which the alloy product contains about 5% to about 50% chromium, up to about 6.5% each of aluminum and titanium, up to about 15% molybdenum, up to about 20% tungsten, up to about 10% columbium, up to about 10% tantalum, up to about 1% silicon, up to about 0.75% carbon, up to about 0.1% boron, up to 1% zirconium, up to about 2% each of manganese and vanadium, up to about 0.3% magnesium, up to about 40% iron, up to about 7% by volume of refractory dispersoid material, and the balance essentially nickel.

References Cited UNITED STATES PATENTS 3,346,427 10/1967 Baldwin, Jr. et al. 148l1.5F 3,366,515 l/1968 Fraser et al. 148-115 F 3,494,807 2/ 1970 Stuart et al. 14811.5 F

WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 

